Portable handheld work apparatus driven by an internal combustion engine include a carburetor for the generation of an air/fuel mixture. An air filter unit for filtering inducted combustion air is mounted on the inflow end of the carburetor. The filter element includes a filter base which is mounted at the carburetor and is connected to the intake end of the carburetor with threaded fasteners. A suction opening is disposed in the filter base and this suction opening communicates with the intake channel of the carburetor. The filter base extends transversely to the longitudinal axis of the suction opening and, in accordance with the state of the art, is provided with a sealing surface at its end facing away from the carburetor. A filter element with a peripherally-extending seal lies against this sealing surface. A clean air side of the air filter unit is formed with a clean air channel because of the filter element, the peripherally-extending sealing ring and the filter base. The carburetor draws filtered combustion air from the clean air channel.
Air filter units of this kind have additional tasks which go beyond the strict filter function. These tasks are grounded in the special performance of single-cylinder two-stroke engines or four-stroke engines used in portable handheld work apparatus.
During operation, the intake air flow is not continuous but pulsates in dependence upon the control times of the internal combustion engine. For a satisfactory running performance, a match of the length of the induction path to the operating rpm of the engine is sought. With a suitable length selection, the pressure pulsations, which occur in the intake path, can be utilized to support the cylinder charge. In conventional constructions, this leads to a large axial structural length which is unwanted in portable handheld work apparatus. The filter element must be arranged at a large distance to the carburetor or to the cylinder of the engine with an axial length adaptation of the intake channel. In contrast, a convenient configuration of the work apparatus with a small volume outer contour is sought so that only limited structural space is available for the length adaptation of the intake path.
The above-described pressure pulsations in the intake channel furthermore lead at low and mid rpms to an effect which is characterized as “back spitting”. Here, during induction, fuel is indeed drawn by suction into the carburetor but is not completely conducted into the cylinder. A back running pressure wave pushes a part quantity of the fuel opposite to the intake direction from the carburetor in the direction of the air filter. This can lead to unwanted collections of fuel and to a wetting of the filter element with fuel or the like. To avoid this problem, arrangements are known with back scattering walls or impact pots which are intended to protect the filter element against back-spat fuel. Here too, the problem of unwanted large axial structural length occurs.